A design to be tested - polytunnel with roof vent.

[Vinlander]

After experiencing the absurd heat levels inside my polytunnel lately I have decided to put in a proper (homemade) roof vent with an automatic opener the next time I change the cover. This benefits from the momentum of the rising hot air and all the authorities agree it can't be beaten. Since I chose the Spanish-made double-stabilised cover it won't need doing for some years. But I have ideas...

The best existing design I've seen is when the tunnels have a "cool input" from a full-length mesh panel about 60cm high at soil level on the shadier side - a strong longitudinal beam forms the border between the mesh and the cover - which is tensioned by being buried the other side of the tunnel.

The problem is that the hot air only gets out when someone opens the doors - and if you don't want to leave them open all day you have to change to mesh doors (or upper-half mesh) at some point in the season. It would make a lot of sense to mesh the transom as well, but I've never seen it.

It occurs to me that I would prefer to have 2 longitudinal beams, one each side of the apex with a vent in between. So both sides of the cover start at a beam and are tensioned by burying. I could then have lower-half mesh doors for a "cool input".

I think it would work much better this way round even if you left out the automatic opener - and that would be crazy!

I'm so happy with this Idea I'm copying it from another thread - I'm not absolutely certain it would work but I think it's worth using Top Tips as a research forum anyway.

Cheers.

PS. Was that a freudian slip in the earlier title?

PPS.  my tunnel uses about 6 metres of cover and over half a metre is wasted - this design would let me buy a metre less cover next time... and the polycarbonate for the vent will last nearly forever.

[Vinlander]

[Tee Gee]

Hi Vin

You might get some ideas at this website!

Have a good look through the site, it is quite big as it caters for both amateur and commercial growers.

I have seen these tunnels/greenhouses at a couple of shows I have attended and if I were starting up again I would certainly have one of these expensive tho they be, but I would have to say they look value for money!

The bottom edges are roll up vents and can be formed of plastic or mesh.

http://www.gardentunnels.co.uk/

[Hector]

Teegee, we have one of the above tunnels and we are delighted with it. Well built, easier to put up than other tunnels...and solid. The roll up mesh and mesh doors are super....so we find less condensation and nicer to work in.
I got mine preowned but if we ever get a second tunnel, it would be one of these.

[Vinlander]

I have looked and I can't see anything new on the Haygrove website at all, certainly no way of including automatic roof vents.

They describe their side vents as unique - I bet they aren't patented - it would be diffcult to get a patent for adding a big rolldown polytunnel door to an ordinary side vent (aaaargh - I hate adverts).

It's obviously an excellent product - but the prices are jaw-dropping. My home-made tunnel cost around 10% of that.

I regard polytunnels as an acceptable cheap substitute for a proper greenhouse (what I really want).

I can't see the point of paying double (actually more) for a no-compromise version of a compromise solution.

I think I can greatly improve the compromise at very little cost by including a roof vent; but...

I'd much prefer the "proper" twinwall polycarbonate greenhouse I could make with proper glazing bars, proper gutters  and 3 proper automatic roof vents for around £500 plus a week's work - say £600 if I wanted it to look good inside too (don't really care). For another £200 I could add 2 or 3 automatic side vents.

Cheers.

[Hector]

Vinlander, the Haygrove has option of door vents too. So air exchange options at sides and ends. You will also read of owners  having innovatively used enviromensh at end panels in very hot conditions.....as this design has separate end panels. Additionally, if you keep an eye out they can be bought for less than the construction costs you mention.....I was able to do that. Each to their own :) they certainly don't have roof vents.

You mention polycarbonate. These look impressive and may give food for thought
http://www.nortenergy.co.uk/our-polycrub/

Edit to add. Ps I'll try and find it but somewhere I've seen aut roll up sides in UK. I think a roof vent that's auto would be better as heat rises....roller to expose mesh built into top of each end.
Is there a way of solar fans to be utilised too? It's often hot plus no breeze when we get good weather.no

[Vinlander]

The polycrub is an interesting design - using the "grain" of the twinwall polycarbonate in line with the hoops of the tunnel means you can do without expensive glazing bars because the stressed skin has its own rigidity. That's a big saving.

The other benefit of using stressed twinwall PC is that it could take a roof vent - it would be strong enough to cope with having an open rectangle in the roof - but since Shetland is 9^o further north I doubt they get any real problem with summer heat!

It's actually very similar to my home-made 1/4 ellipse peach covers that use otherwise fragile acrylic sheet to make the flowers and young fruit visible in the spring (when it's so nice to see them). Unfortunately clear acrylic is sooo expensive and will eventually disintegrate anyway - I wish 1mm clear polycarbonate was easier to source - this design would negate its marked lack of rigidity.

I always wanted a proper "pentagonal" greenhouse but one made from 2 x PC ellipses with a V section at the roofline for the vent (the "gothic" shape) might look even better and be much cheaper to make.

The picture I attached does not mean I endorse this product (whatever it is) - I'm only trying so show how a home-made polycarbonate greenhouse could look much better (and could have much better ventilation than the one shown).